1. Technical Field
The present disclosure relates to a transmitting apparatus and a transmission method for reducing power consumption.
2. Description of the Related Art
To reduce power consumption of a transmitter used in wireless communication, it is generally effective to reduce power consumption of a power amplifier which consumes high power. A class-D amplifier is known as an example of a power amplifier capable of operating with low power consumption. The class-D amplifier is equivalent to a circuit configuration in which one of two switches is disposed between the power supply and the output and she other one is disposed between the ground and the output, and these two switches are alternately turned on such that the power supply voltage and the ground voltage are alternately output. In an ideal operation state, no shoot-through current flows between the power supply and the ground, and thus a high power efficiency is achieved.
However, in practical circuits, rounding of a signal or a timing error may cause the two switches to turn on simultaneously in this situation, a current can flow between the power supply and the ground, which results in a reduction in power efficiency. To handle the shoot-through current, it is known to use a non-overlapping clock technique.
In the non-overlapping clock technique, when the switch disposed between the power supply and the output and the switch disposed between the ground and the output are alternately turned on, a time period (non-overlapping period) is intentionally provided such that the both switches are in the off-state during this time period thereby preventing an occurrence of a shoot-through current that can cause a reduction in the power efficiency. This technique is very effective to increase the efficiency of the class-D amplifier, and thus this technique is widely used.
Next, a method of controlling output power of the class-D amplifier is described below briefly. As described above, the output of the class-C amplifier is ideally connected to a power supply or ground via a switch, such that the output voltage amplitude swings between the power supply and the ground. This makes it possible to control the output voltage by varying the power supply voltage. To vary the power supply voltage, a low-noise and high-response voltage regulator is necessary. However, a power loss occurs in the voltage regulator, and thus use of the voltage regulator results in an increase in power consumption.
In practice, the class-C amplifier is realized using a complementary metal-oxide semiconductor (CMOS) structure. However, the switches in this structure are not ideal but have a finite on-resistance. By changing the sizes of transistors in the CMOS structure, it is possible to change the on-resistance of the switches thereby controlling the output voltage. However, to achieve a sufficiently large variable output power range, it is necessary to vary the on-resistance over a large range from an extremely low value to a great value.
In addition to the technique described above, it also known to use a pulse-width modulation technique. In this technique, the duty ratio of the on-state is varied depending on the output power. Note that the duty ratio is defined by the ratio of an on-period relative to one period of a signal with a constant frequency. In the pulse-width modulation technique, there is a lower limit beyond which a further reduction in the duty ratio is difficult. This makes it difficult to reduce the output, power to a sufficiently low value, and thus it is difficult to achieve a sufficiently wide variable output power range (see, for example, 1999 IEEE High-Efficiency Switched-Mode RF Power Amplifier).
One of known techniques to handle the above situation associated with the controlling of output power is a switched capacitor power amplifier (hereinafter referred to as a SCPA) (see, for example, A Switched-Capacitor RF Power Amplifier, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 46, NO. 12, DECEMBER, 2011). In the SCPA, the above-described problem with the controlling of output power is solved while maintaining the high efficiency property of the class-D amplifier.
For example, as in the amplifier 803 illustrated in FIG. 1, an equivalent circuit of the SCPA includes a plurality of class-D unit amplifiers that are connected in parallel such that the AND output thereof is obtained via a services capacitor. Among class-L) unit amplifiers connected in parallel, as many class-D unit amplifiers as proportional to the output power control signal are turned on, and the AND output of the other class-D unit amplifiers is grounded. In this configuration, the output signal of each D-class unit amplifier in the on-state is transferred to the output side via the corresponding series capacitor, and the output signal of each D-class unit amplifier in the off-state (the AND output of which is grounded) is transferred to the ground via the corresponding series capacitor. That is, the amplitude of the output signal from each D-class unit amplifier in the on-state is transferred to the output side via a capacitive attenuator, and the output power is determined by a capacitance ratio.
In a case where the SCPA is implemented in a semiconductor LSI or the like, it is possible to achieve high relative accuracy among elements in the semiconductor LSI or the like. Therefore, by implementing the SCPA of the above-describe type in a semiconductor LSI or the like, it is possible to achieve very high linearity and a wide variable output power range. Another feature of the SCPA is that when the amplifier 803 is seen from the node Cout 313, which is the output node of the amplifier 803 in FIG. 1, the impedance thereof is constant regardless of the level of the output power control signal. This is because the series capacitor of the AND output is always connected to the ground or the power supply, which can be regarded as being at the ground level for AC signals, regardless of whether the D-class unit amplifier is in the on-state or off-state. As a result, a combination of the series capacitor and an inductor 310 has a constant LC resonance frequency regardless of the output power control signal, which ensures that it is possible to extract a fundamental harmonic component from a signal output from the amplifier 803. Therefore, it is possible to achieve small leakage power to adjacent channels due to an impedance fluctuation, which can occur in the class-A amplifier.
In a transmitting apparatus formed using a plurality of switching circuits such as conventional class-D amplifiers, it is known to use frequency equal to a multiple of the frequency of the local signal in addition to the local signal serving as a carrier wave (see, for example, U.S. Pat. No. 7,750,749).
FIG. 2 is a diagram illustrating a configuration of a transmitting apparatus described in U.S. Pat. No. 7,750,749. In FIG. 2, reference numerals 90, 92, 96, and 98 denote transistors that turn on/off according to local signals input to gates thereof. Reference numerals 90 and 100 denote transistors that turn an/off according to the frequency of signals input to gates thereof wherein the frequency is a multiple of that of the local signal. At voltage transition of signals LOi and LOiB, a signal Vosc is off, while a signal VoscB is off at voltage transition of signals LOq and LOqB. By performing the control in this manner, the effective duty ratio of the local becomes 25%, and the phase of the output signal is determined by the signals Vosc and VoscB, which allows it to achieve low phase noise.
In general, a balun is used to mix I and Q signal components. However, when the balun is formed on an integral circuit, the resultant integral circuit has a large size, which besides results in an increase, in cost. Furthermore, another demerit, thereof is a large power loss in the technique disclosed in “A 45 nm Low-Power SAW-less WCDMA Transmit Modulator Using Direct Quadrature Voltage Modulation”, 2009 ISSCC, like the technique disclosed in U.S. Pat. No. 7,750,749, a signal LO with a duty ratio of 25% is generated using frequency equal to a multiple of the frequency of the local signal (see FIG. 3). Use of the duty ratio of about 25% makes it possible to connect one of switches M1 to M4 to the output side in any state. As a result, it is possible to achieve isolation between I and Q mixers, which makes it possible to directly connect the outputs of I and Q mixers without using a balun such that no interference occurs and good orthogonality is maintained.
As for the quadrature modulator configured to perform IQ mixing without using a balun using the technique disclosed in “A 45 nm Low-Power SAW-less WCDMA Transmit Modulator Using Direct Quadrature Voltage Modulation”, 2009 ISSCC, numerical formula analysis is performed in “Analysis of Direct Conversion IQ Transmitters With 25% Duty Cycle Passive Mixers”, 2011 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS. In this analysis, a mathematical expression shows that a spectrum appears at all, odd order harmonic components of the local signal which is the carrier wave. In the SCPA described above, these odd order harmonic components are removed, using an LC filter formed by the series capacitor connected to the AND output and an inductor 310. As described above, the LC resonance frequency is constant regardless of the output power control signal, and thus it is ensured that it is possible to remove the unnecessary odd order harmonic components other than the fundamental harmonic component.
In the SCPA (disclosed in “A Switched-Capacitor RF Power Amplifier”, 2011 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 46, NO. 12, DECEMBER), a quadrature modulator is formed using two amplifiers 803 in FIG. 1 for an in-phase (I) component and a quadrature-phase (Q) component, respectively, and, as with the quadrature modulator in FIG. 3 according to the technique disclosed in “A 45 nm Low-Power SAW-less WCDMA Transmit Modulator Using Direct Quadrature Voltage Modulation”, 2009 ISSCC, a switch is added in series to the output of each amplifier 803 such that one of I and Q is connected to the output side. In this case, as for a transistor used as the switch to pass the output, power of the SOPA which is a power amplifier, the transistor needs to have a high breakdown voltage and a very low on-resistance. As a result, a very large transistor size is required. However, the large transistor has large parasitic capacitance. As described above, the class-D amplifier is realized using the CMOS structure including P-channel (Pch) and N-channel (Nch) elements such that he output voltage is alternately connected to the power supply and the ground via switches realized by the Pch and Nch elements. Therefore, the large parasitic capacitance causes an occurrence of a charging/discharging current to/from the parasitic capacitance, which results in a reduction in power efficiency of the SCPA. Furthermore, the parasitic capacitance causes the output signal to be rounded and thus distorted.
Note that the SCPA is formed using a plurality of class-D unit amplifiers illustrated in FIG. 4 connected in parallel. As many class-D unit amplifiers as proportional to the output power control signal are turned on (signal BB Data 326=H) and the output voltage thereof is alternately connected to the power supply and the ground via switches. On the other hand, the other class-D unit amplifiers are turned off (signal HF Data 326=L), and the AND output thereof is always grounded unless there is a change in the output power control signal.
However, in the case where the quadrature modulator is formed using two amplifiers 803 in FIG. 1 for the in-phase component and the quadrature-phase component, respectively, the AND output of the amplifier turned off by the output power control signal (signal BB Data 326=L) is always at the ground level even when the local signal with the duty ratio of 25% is used as described in the “A 45 nm Low-Power SAW-less WCDMA Transmit Modulator Using Direct Quadrature Voltage Modulation”, 2009 ISSCC illustrated in FIG. 3. Therefore, when the amplifiers 803 for the in-phase component and the quadrature-phase component are directly connected, the in-phase component and the quadrature-phase component interfere with each other, and thus the orthogonally, which should be achieved, is lost.